Duopole mass filter



D66. 24, 1968 w M U KER ET AL 3,418,464

DUOPOLE MASS FILTER Filed July 27, 1966 INVENTORS. W/zmv M ZF/iA/Ef y Aka/m A. W/z/w United States Patent Office 3,418,454 Patented Dec. 24, 1968 3,418,464 DUOPOLE MASS FILTER Wilson M. Brubaker, Arcadia, and Frank B. Wiens,

Altadeua, Calif., assignors to Bell & Howell Company, Chicago, 111., a corporation of Illinois Filed July 27, 1966, Ser. No. 568,264 7 Claims. (Cl. 25041.9)

ABSTRACT OF THE DISCLOSURE A non-magnetic mass analyzer utilizing only two field electrodes and a single planar reference electrode to define an analyzing field therebetween. One or more sources of charged particles are located at the entrance end of the filter and a planar collector electrode is located at the opposite end of the filter. AC and DC voltages of a predetermined polarity are connected to the field electrodes and a reference voltage is connected to the planar reference electrode resulting in the creation of a mass analyzing electric field between the field electrodes and the reference electrode.

This invention relates to mass filters and in particular to filters used as mass analyzers operating on the monopole mass filter principle.

In US. Patent 3,197,633, there is described a method and apparatus for separating ions of different specific electric charges. This apparatus is commonly referred to as a monopole mass filter or mass analyzer. Such an apparatus is comprised of an elongated electrode or rod and a right angle electrode located adjacent the rod and spaced a predetermined distance therefrom. The filter is energized by connecting a source of a varying voltage and a source of a fixed voltage to the elongated rod and a source of reference voltage to the right angle electrode. By proper choice of amplitude and frequency of the energizing voltages it has been found that ions admitted into the space between the rod and the right angle electrode assume stable and unstable trajectories, depending on their mass-to-charge ratio.

As is described in more detail in 3,197,633, the monopole is similar in principle to a quadrupole mass analyzer with the exception that it has only one rod. The grounded right angle electrode which the single rod faces images the effect of the missing quadrupole rods causing charged particles admitted into the filter to encounter electric fields which are identical to those in a quadrupole mass filter. Under the same conditions of operation, the trajectories of charged particles such as ions are the same in a monopole mass filter as in a quadrupole mass filter. The monopole differs from the quadrupole in that it is not operated with the same parameters, the parameters for the former differing from those of the latter in such particulars as rod size, and amplitude and frequency of energizing voltages.

Another important diiference between the quadrupole and a monopole is in the manner in which the so-called x-component of motion is treated. In the quadrupole the transmission of ions whose x-component of motion is unstable is eliminated by locating the working point of the filter in the x-unstable portion of the mass filter stability diagram. In contrast, in the monopole the transmission of ions is not a function of the stability conditions for the x-component of motion. With the monopole the right angle or reference electrode acts as a barricade which causes ions to be lost from the analyzer even when the x-component of motion is stable.

Another difference is that in the quadrupole, ions whose trajectories are stable for the y-component of motion may be lost to the x-rods because its -x-component of motion becomes unstable and its amplitude grows exponentially with time. This problem is eliminated in the monopole because the x-component of motion of ions remains quite stable. Under these conditions, the amplitude of motion in the x-direction is small and the ions remain in a fairly thin sheet, the plane of which passes through the rod axis and the apex of the grounded or reference electrode.

A problem encountered in the operation of the monopole mass filter is the susceptibility of the ion collector to induction of a potential from the energizing field of the filter. Because the monopole is by its nature unbalanced, the collector is electrically coupled to the filter at high voltage and high frequency, thereby making detection of charged particles impinging thereon difiicult where the collector is a simple planar electrode surface, which is connected directly to the input grid of an electrometer amplifier. Of necessity the operation of the monopole imposes the necessity of using a secondary emission multiplier in order to enable the detection of particles passed by the filter.

The present invention provides a variation of the conventional monopole mass filter structure by providing an apparatus suitable for use as a mass'filter which comprises at least one source of charged particles and a corresponding number of collector means for detecting such particles located a spaced distance away from the source along a common axis. A first and second electrode are disposed adjacent the axis between the charged particle source and collector means. The two electrodes are disposed in a generally parallel orientation relative to each other and to the common axis of the apparatus. The apparatus further includes a source of AC voltage, a source of DC voltage, first circuit means for connecting the AC source and the DC source to the first electrode with a predetermined polarity and second circuit means for connecting the AC source and the DC source to the second electrode, the voltages connected to the second electrode having a polarity opposite the polarity of the voltages connected to the first electrode.

As previously indicated, in a conventional monopole mass filter the detector is a secondary emission multiplier. By virtue of the balanced or dual rod configuration of the present invention, with the application of equal and opposing potentials to the two rods, the potential that would otherwise be induced at the collector by the coupling between one of the rods and the collector is balanced and neutralized by the opposing potentials coupled between the second rod and the collector, leaving the collector with a zero potential induced thereon. This balanced configuration means that it is possible to eliminate the use of the secondary emission multiplier and replace the collector with the simple planar collector electrode. These and other advantages of the present invention will be better understood by reference to the following figures in which:

FIG. 1 is a diagrammatic and perspective representation of a conventional monopole assembly;

FIG. 2 is a diagrammatic representation of the mirror image rod presented to the rod of the conventional monopole filter;

FIG. 3 is a schematic representation of a filter in which an actual rod is positioned in the place of the image rod of FIG. 2; and

FIG. 4 is a schematic and perspective view of the filter of the present invention used as a mass filter.

Referring now to FIG. 1, there is shown therein a conventional monopole mass filter comprising an elongated field electrode 10 and a right angle reference electrode 12.

The field electrode is disposed and supported at a predetermined distance from the two planes 14 and 16, respectively, of the reference electrode 12. The reference electrode is connected to a source of reference voltage 18, normally ground potential, and the field electrode 10 is connected to a source of varying voltage and a source of fixed voltage 22 such that an analyzing electric field is created in the space between electrodes 10 and 12. The polarity of the DC source 22 is selected in accordance with the charge of the particles to be analyzed. For negatively charged particles, the positive pole of the DC source is connected to the rod; for positively charged particles the negative pole is connected to the rod. Particles introduced into the space between the two electrodes assume stable or unstable trajectories, depending on their mass-to-charge ratio, and all but those having a mass-to-charge ratio of interest are lost at the reference electrode or discharged at the field electrode.

The configuration and energization of the conventional monopole filter has the effect of causing the single field electrode of the monopole to experience or see the same electric fields as if this electrode were part of a quadrupole mass filter. This effect is illustrated in FIG. 2 wherein an image rod 24 disposed alongside of rod 10 and separated therefrom by reference electrode 12 is shown. The electric fields created within the monopole in its conventional configuration cause ions admitted into the filter to react as though a rod 24 excited by varying and fixed voltages of opposite polarity to that connected to rod 10 were present.

The present invention provides an apparatus as shown in FIG. 3. In that figure a rod 26 disposed in a generally parallel relationship to a second rod 28 defines an area wherein the static and alternating field components can be created, thereby enabling the analysis and separation according to the mass-to-charge ratio of the charged particles admitted therein. The vertical portion 14 of right angle reference electrode 12 in FIG. 1 is eliminated and the reference electrode is now a planar surface 30 disposed beneath rods 26 and 28. A source of DC voltage 32 and AC voltage 34 are connected to rod 26; similarly, an AC source 36 and a DC source 38 are connected to rod 28. The polarity of all of the sources is chosen such that those connected to rod 26 are opposite in polarity to those connected in rod 28. Reference electrode 30 is connected to a source of reference voltage 40, such as ground. The result is essentially a duopole filter.

In FIG. 4 is shown one way in which a duopole filter can be used as a mass analyzer. In that figure sources of charged particles 42 and 43 are arranged at one end of a pair of rods 26 and 28 and oriented so as to direct charged particles along axes 44 and 45 toward collectors 46 and 47. The collectors 46 and 47 are shown as a planar electrode and, as indicated above, the use of this type of electrode is a prime advantage of this apparatus over conventional monopoles because of the elimination of induced voltages in the collector. Ions emerging from the exit of the filter impinge upon the collectors 46 and 47 and signals are conducted to amplifiers 48 and 49 and thence to recorders 50 and 51 such as strip chart recorders.

As is typical of a conventional quadrupole structure, the apparatus of this invention is also energized with symmetrical AC and DC potentials with the DC component being restricted to a value less than 17% of the peak AC component. This restriction on the DC component restrains the ratio of the DC to AC voltage to a value of less than 0.169, the voltage ratio at which the filter theoretically has infinite resolution.

The methods and apparatus of the present invention retain one of the significant features of the conventional monopole filter because it provides a mass analyzer capable of achieving charged particle separation or mass spectrographic analysis wit-h an apparatus of simple design and minimal attendance requirements. In addition, it provides a significant advantage over the conventional monopole in that it is capable of analyzing both positively and negatively charged particles simultaneously. In such a simultaneous analysis positively charged particles are directed into the filter adjacent the rod connected to the negative side of a DC source and negatively charged particles adjacent the rod connected to the positive side of a DC source.

What is claimed is:

1. A duopole mass filter consisting of:

at least one source of charged particles;

collector means for detecting charged particles, the source and collector means being located along a common axis;

a first rod electrode disposed along and spaced a predetermined distance from the common axis between the source and the collector means;

a second rod electrode disposed along and spaced a predetermined distance from the common axis between the source and collector means, the second electrode being located in a parallel orientation relative to the first electrode;

an auxiliary planar electrode disposed adjacent the first and second electrode on the side of the common axis opposite said first and second electrodes and oriented substantially parallel to the plane of said first and second electrodes such that said first and second electrodes and said auxiliary electrode generally surround said axis;

a source of reference voltage connected to the auxiliary electrode;

a source of varying voltage;

a source of fixed voltage;

first circuit means for connecting the source of varying voltage and the source of fixed voltage between the first electrode and the auxiliary electrode, the varying and fixed voltages having a predetermined polarity; and

second circuit means for connecting the source of varying voltage and the source of fixed voltage between the second electrode and the auxiliary electrode, the polarity of the varying and fixed voltages connected to said second electrode being opposite the polarity of the voltages connected to the first electrode.

2. Apparatus according to claim 1 wherein the first and second electrodes present a substantially convex surface to the auxiliary planar electrode.

3. Apparatus according to claim 2 wherein the collector means is a planar electrode.

4. Apparatus according to claim 2 wherein the first and second electrodes are cylindrical rods.

5. Apparatus according to claim 1 wherein the source of varying voltage is a source of a radio frequency AC voltage and the source of fixed voltage is a source of DC voltage.

6. Apparatus according to claim 5 wherein the amplitude of the voltage from the DC source is less than 17% of the peak value of the voltage from the AC source.

7. A non-magnetic duopole mass filter utilizing only two field electrodes and a single reference electrode to create an analyzing field about an analyzer axis, said two field electrodes having a rod configuration and being dis posed parallel to each other and parallel to the analyzer axis and the reference electrode having a planar configuration and being disposed parallel to a plane defined by the longitudinal axes of said field electrodes and parallel to said analyzer axis on the side of said axes opposite the rod electrodes such that said two field electrodes and said reference electrode generally suround said axis;

a source of charged particles located at one end of the filter; planar collector electrode means located at the end of the filter opposite the source of charged particles; a source of reference voltage connected to the reference electrode;

5 6 a source of AC voltage; ing a polarity opposite the polarity of the DC voltage a source of DC voltage; connected to the first field electrode. first circuit means connecting the source of AC voltage and the source of DC voltage between one of said References Cited field electrodes and the reference electrode, the fixed 5 voltage having a predetermined polarity; and UNITED STATES PATENTS second circuit means connecting the source of AC volt- 2,939,952 6/ 1960 P l t 1 25() 41 9 age and the source of DC voltage between the second 3,280,326 10/ 1966 G th 250 41 9 field electrode and the reference electrode, the fixed voltage connected to said second field electrode hav- 10 WILLIAM F. LINDQUIST, Primary Examiner. 

